High elastic modulus polymer electrolytes suitable for preventing thermal runaway in lithium batteries

ABSTRACT

A polymer that combines high ionic conductivity with the structural properties required for Li electrode stability is useful as a solid phase electrolyte for high energy density, high cycle life batteries that do not suffer from failures due to side reactions and dendrite growth on the Li electrodes, and other potential applications. The polymer electrolyte includes a linear block copolymer having a conductive linear polymer block with a molecular weight of at least 5000 Daltons, a structural linear polymer block with an elastic modulus in excess of 1×10 7  Pa and an ionic conductivity of at least 1×10 −5  Scm −1 . The electrolyte is made under dry conditions to achieve the noted characteristics. In another aspect, the electrolyte exhibits a conductivity drop when the temperature of electrolyte increases over a threshold temperature, thereby providing a shutoff mechanism for preventing thermal runaway in lithium battery cells.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of the internationalapplication PCT/US2007/008435 filed on Apr. 3, 2007, titled HIGH ELASTICMODULUS POLYMER ELECTROLYTES; which claims priority to U.S. ProvisionalPatent Application No. 60/744,243 filed Apr. 4, 2006, titled HIGHELASTIC MODULUS POLYMER ELECTROLYTES; and to U.S. Provisional PatentApplication No. 60/820,331 filed Jul. 25, 2006, titled SYNTHESIS OF DRYPOLYMER ELECTROLYTES. The disclosures of applications listed above areincorporated herein by reference in their entirety and for all purposes.

STATEMENT OF GOVERNMENTAL SUPPORT

This invention was made with government support under ContractDE-AC02-05CH11231 awarded by the United States Department of Energy toThe Regents of the University of California for the management andoperation of the Lawrence Berkeley National Laboratory. The governmenthas certain rights in this invention.

FIELD OF THE INVENTION

The invention relates to polymer electrolytes. More particularly, theinvention relates to high elastic modulus, high ionic conductivitypolymer electrolytes comprising linear block copolymers, and methods ofmaking them. The invention further relates to methods for preventingthermal runaway in lithium batteries.

BACKGROUND OF THE INVENTION

Polymer membranes with high ionic conductivity are important forapplications such as solid-state batteries and fuel cells. Theperformance of these materials depends not only on their electricalproperties but also on other properties such as shear modulus,permeability, and the like. The mechanical properties of polymerelectrolytes are particularly important in secondary solid-state lithium(Li) batteries. One of the challenges in the field of rechargeable Liion batteries is to combine high energy density with good cyclabilityand electrode stability. Batteries that employ Li metal anodes for highenergy density applications suffer from failures due to side reactionsand dendrite growth on the Li electrodes. Repeated cycling of thebatteries causes roughening of the Li surface and eventually to dendriteformation, which reduces battery life and compromises safety.

Recent theoretical work indicates that dendrite growth can be stopped ifthe shear modulus of current polymer electrolytes can be increased bythree orders of magnitude without a significant decrease in ionicconductivity. Other studies have shown that cation transport isintimately coupled to segmental motion of the polymer chains. Thesestudies indicate that dendrite growth on the electrode surface can beprevented by introducing a highly rigid electrolyte (elastic modulus ofabout 1 GPa) between the two electrodes. This high modulus requirementessentially renders most rubbery polymer electrolytes incompatible withthe electrode material, as the elastic moduli of typical rubberypolymers are about 1 MPa. For example, poly(ethylene oxide) (PEO) melt,one of the most studied polymer electrolytes, has an elastic modulus ofless than 1 MPa. High ionic conductivity is obtained in soft polymerssuch as PEO because rapid segmental motion needed for ion transport alsodecreases the rigidity of the polymer. Glassy polymers such aspolystyrene offer very high moduli (about 3 GPa) but are poor ionconductors. Thus, conductivity and high modulus have appeared to bealmost mutually exclusive goals in polymer electrolytes.

There is, therefore, a need to develop a new methodology for decouplingthe electrical and mechanical properties of polymer electrolytematerials. Such a material would be useful as a solid phase electrolytefor high energy density, high cycle life batteries that do not sufferfrom failures due to side reactions and dendrite growth on the Lielectrodes.

A separate problem encountered during operation of rechargeable lithiumbatteries is thermal runaway characterized by uncontrolled heating ofthe battery cell during operation (e.g., during charging). Suchuncontrolled heating occurs because of a positive feedback whichgenerally exists between cell temperature and conductivity of theelectrolyte in a battery. As the temperature of the cell rises, theconductivity of electrolyte increases, leading to additional increase incell temperature, leading, in turn, to an increase in electrolyteconductivity, and so on. When uncontrolled, this cycle can lead tooverheating (thermal runaway) of the cell, which can cause melting oflithium metal and violent chemical reactions. Thermal runaway is aserious safety problem in lithium battery design, which is currentlyaddressed by complex engineering solutions. Development of additionalmethods for preventing thermal runaway in rechargeable lithium batteriesis desirable.

SUMMARY OF THE INVENTION

The present invention provides a polymer that combines high ionicconductivity with the structural properties required for Li electrodestability. The polymer is useful as a solid phase electrolyte for highenergy density, high cycle life batteries that do not suffer fromfailures due to side reactions and dendrite growth on the Li electrodes,and other potential applications. In one aspect, the polymer electrolyteincludes a linear block copolymer having a conductive linear polymerblock with a molecular weight of at least 5000 Daltons, a structurallinear polymer block with an elastic modulus in excess of 1×10⁷ Pa andan ionic conductivity of at least 1×10⁻⁵ Scm⁻¹. The electrolyte is madeunder dry conditions to achieve the noted characteristics. Theelectrolyte includes a lithium salt dissolved within the conductivelinear block of the block copolymer.

In specific embodiments, the linear block copolymer is a diblockcopolymer characterized by bicontinuous lamellar phases of the polymerblock constituents. In some embodiments the structural polymer block canform a continuous rigid framework through which the conductive polymerblock forms continuous nanostructured ionically conductive channels. Inone embodiment, the structural polymer block occupies at least 50%,e.g., at least 60% of block copolymer by volume, while the conductiveblock forms cylindrical nanostructure embedded within the rigidframework of the structural block. Notably, in some embodiments thestructural block is a glassy polymer in a large temperature range (e.g.,up to 90° C., or even up to 100° C.). In some embodiments, the entireblock copolymer is effectively glassy at least to a temperature of up to70° C.

In one aspect, the invention relates to a polymer electrolyte. Theelectrolyte includes a linear block copolymer having a conductivepolymer block with a molecular weight of at least 5000 Daltons, astructural polymer block with an elastic modulus in excess of 1×10⁷ Pa,and an ionic conductivity of at least 1×10⁻⁵ Scm⁻¹.

In another aspect, the invention relates to a polymer electrolyte whichis suitable for preventing thermal runaway in a lithium battery cell. Itwas unexpectedly discovered that conductivity of certain polymerelectrolytes doped with lithium salts (specifically, block copolymers)decreases once the temperature increases above a threshold value. Thesepolymer electrolytes may be suitable for preventing thermal runaway inlithium battery cells. Specifically, in some embodiments, the thresholdtemperature at which the conductivity drop occurs is between 90° C. and150° C., e.g., between 100° C. and 120° C. In some embodiments theconductivity drop is at least 5-fold, preferably at least 10-fold. Thisconductivity drop is due to precipitation of the lithium salt in theblock copolymer, which, unexpectedly occurs with temperature increase.Accordingly, in some aspects, a block-copolymer configured fordissolving a lithium salt at a first temperature and precipitating thissalt at a higher temperature, wherein the precipitation is accompaniedwith a conductivity drop, is provided.

In another aspect, the invention relates to a method of making a polymerelectrolyte. The method involves, in an oxygen and moisture freeenvironment, forming a linear block copolymer having a conductivepolymer block with a molecular weight of at least 5000 Daltons and astructural polymer block with an elastic modulus of at least 1×10⁷ Paand incorporating a Li salt into the linear block copolymer such thatthe resulting polymer electrolyte has an ionic conductivity of at least1×10⁻⁵ Scm⁻¹.

In some embodiments, the method of making the polymer electrolytefurther involves heating the block copolymer having the lithium saltdissolved therein in a casting and/or annealing process, to form thepolymer electrolyte film. In some embodiments the casting and/orannealing is performed at a temperature, at which lithium salt does notprecipitate. For example, for those block copolymers which exhibit saltprecipitation with increase of temperature, the film may be heated at atemperature that is below the threshold temperature from the range of90° C. to 150° C. The threshold temperature at which the saltprecipitation occurs therefore, is an important parameter, which isconsidered for determining the optimal temperature range for copolymerfilm processing. In some embodiments, casting and/or annealing isperformed at a temperature above the glass transition temperature of thestructural block of the copolymer but below the threshold temperature atwhich salt precipitation occurs.

In a further aspect, the invention relates to battery cells comprising aLi anode, a cathode and linear block copolymer electrolyte in accordancewith the invention disposed between the anode and cathode. The cell canbe cycled without detrimental dendrite growth on the anode. In oneembodiment, the battery cell includes a linear block copolymerelectrolyte which exhibits a drop in conductivity once the temperatureexceeds a threshold temperature. In some embodiments, the thresholdtemperature is in a range from 90° C. to 150° C., and the conductivitydrop is at least 2-fold, or at least 5-fold, and, preferably, at least10-fold. Such polymer electrolytes can be used to prevent thermalrunaway in a lithium metal battery.

In a further aspect, the invention relates to a method of operating arechargeable lithium cell. The method involves providing a battery cellhaving a lithium anode, a cathode, and a solid block copolymerelectrolyte having a lithium salt dissolved therein, wherein the blockcopolymer electrolyte is configured for precipitating the dissolvedlithium salt with increase of temperature above a threshold temperature.The method includes charging and discharging the battery cell, with thecell being configured for cooling or reduced heating once theelectrolyte reaches the threshold temperature. The cooling or reducedheating is due to conductivity drop in the electrolyte which is causedby lithium salt precipitation.

In a further aspect, the invention provides a method for screening blockcopolymer electrolytes for those which are suitable for preventingthermal runaway in a battery cell. The screening method involvesproviding a plurality of block copolymers, having a lithium saltdissolved therein; measuring dependence of conductivity versustemperature for at least some of the provided block copolymers; and,based on the obtained measurements, identifying the block copolymerswhich exhibit a drop in conductivity with increase of temperature abovea threshold temperature. For example, block copolymers exhibiting a5-fold, preferably 10-fold decrease in conductivity, after thetemperature is increased above the threshold temperature from the rangeof between 90° C. and 150° C., may be selected for use in battery cellsconfigured for thermal runaway shutoff.

These and other aspects and advantages of the present invention aredescribed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of this specification, illustrate one or more embodiments and,together with the detailed description, serve to explain principles andimplementations of the invention.

In the drawings:

FIG. 1A is an illustration of a block copolymer in accordance with thepresent invention in isolation, showing the linearity of the polymerblock chains.

FIG. 1B depicts the chemical structure of a polystyrene-b-poly(ethyleneoxide) (PS-b-PEO, also referred to as SEO) diblock in accordance withone embodiment of the present invention.

FIGS. 2A-C illustrate embodiments of a PS-b-PEO diblock.

FIG. 3 is a graph illustrating Small Angle X-ray Scattering (SAXS)profiles obtained from SEO/salt and SIEO/salt mixtures in accordancewith the present invention.

FIG. 4A is a graph illustrating the dependence of ionic conductivity onr (Li salt per EO unit) and temperature in SEO (36-25), a blockcopolymer electrolyte in accordance with the present invention.

FIG. 4B is a graph illustrating the dependence of ionic conductivity onr (Li salt per EO unit) in SEO (74-98), a block copolymer electrolyte inaccordance with the present invention.

FIGS. 5A-5C are plots illustrating the dependence of ionic conductivityof block copolymer electrolytes in accordance with the present inventionon the molecular weight of their respective PEO blocks.

FIG. 6 is a graph illustrating the frequency dependence of the storageand loss shear moduli of SEO(36-25), a block copolymer electrolyte inaccordance with the present invention, with and without salt, and PEO.

FIG. 7 is a graph illustrating the frequency dependence of elasticmoduli for the series of copolymers referenced in FIG. 3.

FIG. 8 is a plot of the results of DC cycling of a Li/SEO/Li cell withan electrolyte in accordance with the present invention.

FIG. 9A is a bright field electron micrograph of a block copolymer inaccordance with the present invention.

FIG. 9B is a lithium energy filtered electron micrograph of the sameblock copolymer region as FIG. 9A.

FIG. 10 presents data showing lamellae thicknesses for block copolymersin accordance with the present invention obtained from variousmeasurements.

FIG. 11 is a plot of Li lamellae thickness normalized by the PEOlamellae thickness as a function of molecular weight.

FIG. 12 is a plot comparing the normalized Li lamellae thickness andnormalized conductivity versus molecular weight for various blockcopolymers in accordance with the present invention.

FIG. 13 is a simple schematic diagram of a battery cell in accordancewith the present invention.

FIG. 14 is a plot illustrating the dependence of conductivity on inversetemperature for a SEO (54-23) block copolymer. A drop in conductivityabove a threshold temperature is illustrated.

FIG. 15 is a SAXS plot illustrating precipitation of lithium salt in aSEO (54-23) block copolymer with increase in temperature.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Reference will now be made in detail to specific embodiments of theinvention. Examples of the specific embodiments are illustrated in theaccompanying drawings. While the invention will be described inconjunction with these specific embodiments, it will be understood thatit is not intended to limit the invention to such specific embodiments.On the contrary, it is intended to cover alternatives, modifications,and equivalents as may be included within the spirit and scope of theinvention as defined by the appended claims. In the followingdescription, numerous specific details are set forth in order to providea thorough understanding of the present invention. The present inventionmay be practiced without some or all of these specific details. In otherinstances, well known process operations have not been described indetail in order to not unnecessarily obscure the present invention.

It will, of course, be appreciated that in the development of any actualimplementation of the invention, numerous implementation-specificdecisions must be made in order to achieve the developer's specificgoals, such as compliance with application- and business-relatedconstraints, and that these specific goals will vary from oneimplementation to another and from one developer to another. Moreover,it will be appreciated that such a development effort might be complexand time-consuming, but would nevertheless be a routine undertaking ofengineering for those of ordinary skill in the art having the benefit ofthis disclosure.

Introduction

As noted above, in one aspect, the present invention provides a polymerthat combines high ionic conductivity with the structural propertiesrequired for Li electrode stability. The polymer is useful as a solidphase electrolyte for high energy density, high cycle life batteriesthat do not suffer from failures due to side reactions and dendritegrowth on the Li electrodes, and other potential applications. Thepolymer electrolyte includes a linear block copolymer having aconductive linear polymer block with a molecular weight of at least 5000Daltons, a structural linear polymer block with an elastic modulus inexcess of 1×10⁷ Pa and an ionic conductivity of at least 1×10⁻⁵ Scm⁻¹.In some embodiments, the block copolymer has an elastic modulus ofgreater than 100 MPa, e.g., 300-700 MPa. The electrolyte is made underdry conditions to achieve the noted characteristics.

In specific embodiments, the linear block copolymer is a diblockcopolymer characterized by bicontinuous lamellar phases of the polymerblock constituents, and the structural polymer block can form a rigidframework in which the conductive polymer block forms nanostructuredionically conductive channels. In other embodiments, the conductiveblock forms a cylindrical nanostructure within the rigid frameworkformed by the structural block.

Linear Block Copolymers

A linear block copolymer can be used to produce a polymer electrolytethat can combine high ionic conductivity with the structural propertiesrequired for Li metal electrode stability. The electrolyte is useful asa solid phase electrolyte for high energy density, high cycle lifebatteries that do not suffer from failures due to side reactions anddendrite growth on the Li electrodes. The linear block copolymer may bea composite polymer electrolyte having relatively soft (e.g., having anelastic modulus on the order of 1 MPa or less) nanoscale conductingchannels embedded in a relatively hard (e.g., having an elastic moduluson the order of 1×10⁷ Pa or more) polymer matrix that need not beionically conductive. The resulting block copolymer preferably has anelastic modulus of at least 100 MPa, e.g., 300-700 MPa. Ionicconductivity in the channels is conferred by a Li salt incorporated withthe soft polymer block. It has been found that particular configurationsand fabrications of such linear block copolymers enable the combinationof high ionic conductivity and high elastic modulus in a polymerelectrolyte.

While prior work in the field has suggested block copolymers as suitablematerials for electrolytes, these prior block copolymers were composedof relatively soft (non-glassy) polymer blocks that were modified withvarious side chains.

The electrolytes of the present invention, however, comprise blockcopolymers with linear polymer block constituents, one of which is softand conductive to Li ions; another of which is hard and need not beionically conductive. FIG. 1A provides an illustration of such acopolymer in isolation, showing the linearity of the polymer blockchains.

The inventive polymer electrolytes, in one aspect, include a linearblock copolymer having a conductive linear polymer block with amolecular weight of at least 5000 Daltons, a structural linear polymerblock with an elastic modulus in excess of 1×10⁷ Pa and an ionicconductivity of at least 1×10⁻⁵ Scm⁻¹. The polymer, in some embodiments,is non-crosslinked. Remarkably, even without crosslinking high elasticmodulus values of at least 100 MPa are achieved, due to rigidity of thestructural block. In other embodiments, the structural block of thecopolymer is cross-linked, while the conductive block remainsnon-crosslinked.

In a specific embodiment, the copolymer is a linear diblock copolymer ofpoly(ethylene oxide) (a rubbery, linear polymer, highly ionicallyconductive when embedded with an appropriate Li salt) with polystyrene(a non-conductive, glassy, linear polymer); apolystyrene-b-poly(ethylene oxide) (PS-b-PEO) diblock, the chemicalstructure of which, is illustrated in FIG. 1B. Notably, both polystyrene(PS) and polyethyleneoxide (PEO) blocks are linear and are covalentlyconnected with one another without forming branches. This combinationhas been found to provide both the high ionic conductivity and highrigidity sought after for high performance solid state polymerelectrolytes. It is understood that the SEO polymer can have a varietyof terminating groups instead of illustrated sec-Butyl group at thepolystyrene terminus and OH at the PEO terminus. For example, a varietyof alkyl groups can be introduced at the polystyrene terminus (such asmethyl, ethyl, propyl, i-propyl, n-butyl, etc.). The PEO terminus, insome embodiments, contains an alkoxy terminal group (such as methoxy,ethoxy, propoxy group, etc.). In some embodiments alkoxy-terminatedpolymers are preferable over hydroxy-terminated polymers, due to thelower reactivity of the former.

FIGS. 2A-C illustrate embodiments of a PS-b-PEO diblock in accordancewith the invention. The PS-b-PEO diblock, generally numbered 10,exhibits an extensive phase behavior, which allows for bicontinuousphases. In these embodiments, in the bicontinuous PS-b-PEO phases, themajor (e.g., greater than 50%) phase (PS) 12 provides a rigid frameworkfor the nanostructured ionically conducting channels (PEO) 14.

FIGS. 2A-C illustrate the shape and placement of polymer blockscomprising conceptual diblocks in accordance with the present invention,and are not meant to be limiting. It will be known and understood thatthe microstructure of the diblock may be in any shape that is necessaryand the placement of the polymers within the diblock may vary as needed.As schematically illustrated in the FIG. 2A, the cylindrical PEOchannels 14 may be positioned in an orderly placement within the PSmatrix 12. The nanostructure, shown in FIG. 2A is referred to ascylindrical nanostructure. Alternatively, as illustrated in FIG. 2B, thePEO channels 14 may be branched, such as in a gyroid phase, within thePS matrix 12. FIG. 2C illustrates another specific embodiment in whichthe PEO channels 18 are layered in a lamellar arrangement with the PSmatrix 16. While it has previously been believed that networkmorphologies are essential for ion transport, it was surprisingly foundthat diblock copolymers in accordance with the present invention withsuch non-network lamellar morphologies are very effective iontransporters.

In many instances, the diblock copolymer has a non-conducting phase thatis continuous, and, in most cases, the major component, which differsfrom previously proposed block copolymer electrolytes. The mechanicalproperties of such block copolymer electrolytes are dominated by thoseof the non-conducting component. This enables independent control overthe electrical and mechanical properties of the electrolyte. Thestructural linear polymer block major phase forms a rigid framework,with the conductive polymer block forming nanostructured ionicallyconductive channels through the rigid framework.

In specific embodiments, the linear block copolymer is a diblockcopolymer and may be characterized by bicontinuous phases of the polymerblock constituents such that there are continuous ionically-conductivechannels (or pathways) through a continuous rigid framework. In thisway, the beneficial properties of both polymer block materials(structural and conductive) exist throughout the material. Blockcopolymers are known in the art to self-assemble into a variety ofstructures or arrangements. As noted above, a particular arrangement fordiblock copolymers in accordance with the present invention withadvantageous properties is lamellar, although other arrangements arepossible.

An important feature of the block copolymers is the combination of softand hard polymer blocks to form a rigid material that is also highlyionically conductive. Thus, the structural polymer block has an elasticmodulus of at least 1×10⁷ Pa, and generally from about 1×10⁷ Pa to 3GPa. A suitable polymer for the structural block is polystyrene, aglassy polymer with an elastic modulus of about 3 GPa. Other mechanismsfor obtaining suitable rigid frameworks include cross-linking andcrystallization of structural blocks. The ionically conductive polymer,on the other hand, typically has an elastic modulus of no more than 1MPa; poly(ethylene oxide), a specific material for which advantageousproperties have been found in block polymers of the present invention,has an elastic modulus of less than 1 MPa. The resulting block copolymerin some embodiments has an elastic modulus of at least 100 MPa. Further,the resulting block-copolymer, in some embodiments, is glassy at arelatively large temperature range (e.g., up to 70° C., and even up to90° C.).

Other examples of polymers suitable as conductive or structural polymersfor these block copolymer electrolytes include poly vinyl pyrrolidine(conductive), poly acrylates (conductive), polyvinylcyclohexane(structural), epoxies (structural), and polypropylene (structural).

As noted above, the conductive polymer block chains of the blockcopolymer have a molecular weight of at least 5000 Daltons. This isnotable since prior work in the field has indicated an inverserelationship between the molecular weight of conductive polymers andtheir ionic conductivity. To the contrary, it has been found that theionic conductivity of the block copolymers of the present inventionincreases with increasing molecular weight of the constituent polymerblocks. Accordingly, in various embodiments, the structural polymer hasa molecular weight of at least 5000 Daltons. Further, in variousembodiments, the ionic conductivity of the block copolymer increaseswith increasing molecular weight of the conductive polymer block, so theconductive and structural polymer block chains have molecular weights ofat least 15,000 Daltons; or at least 20,000 Daltons; or at least 50,000Daltons; or up to about 100,000 Daltons or more. In some embodiments,the molecular weight of the structural block is at least 200,000, e.g.,or at least 300,000. Ionic conductivity of greater than 1×10⁻⁴ Scm⁻¹ andup to about 1×10⁻³ Scm⁻¹ has been achieved with these relatively highmolecular weight block copolymers in accordance with the presentinvention.

As noted above, ionic conductivity is achieved for the conductivepolymer block by incorporating an appropriate Li salt into theconductive polymer block. It is important that incorporation of the Lisalt be done in a controlled environment and under conditions selectedto achieve the desired performance in the resulting polymer electrolyteproduct. This is discussed further below. Suitable Li salts includelithium bis(pentafluoroethane sulfonyl) imide, lithiumbis[1,2-oxalato(2-)—O,O′]borate (LiBOB), and lithiumbis(trifluoromethane sulfonyl) imide (LiN[SO₂CF₃]₂, also known asLiTFSI) LiClO₄, LiPF₆, LiAsF₆, LiSO₂CF₃ (also known as lithiumtriflate), and others known in the art. One specific salt isLiN[SO₂CF₃]₂ (also known as LiTFSI). Self-assembly of the conductivepolymer block results in ionically conductive channels in the rigidstructural polymer block framework. The salts and the polymers areselected such that the lithium salt is capable of dissolving within theconductive portion of the block copolymer. For example LiTFSI has asolubility limit of 56 wt % in PEO. The concentration of lithium saltwithin the block copolymer is also given by a ratio of molarconcentration of lithium ion per molar concentration of conductive blockmonomer unit. For example, for SEO block copolymers, the concentrationof lithium salt is given by r=[Li]/[EO], where EO is ethylene oxidemonomer unit. In some embodiments r ranges from 0.02 to 0.12, e.g., from0.05 to 0.1.

Further, as also described below, it has been found that the enhancedionic conductivity obtained with increasing molecular weight of theconductive polymer block is correlated with a concentration of Li saltin a central portion of the ionically conductive channels in the blockcopolymer electrolytes.

While many specific embodiments of the invention exhibit a diblockcopolymer structure, other structures are also possible. For example, anadditional polymer block may be added to adjust the properties of theresulting copolymer, in this case a triblock copolymer. One suitabletriblock copolymer is a polystyrene-block-polyisoprene-blockpoly(ethylene oxide) (S—I-EO) triblock copolymer. For example, a S—I-EOtriblock copolymer with molecular weights of 11,000 Daltons for thepolystyrene block, 6000 Daltons for the polyisoprene block, and 9000Daltons for the poly(ethylene oxide) block was found to have a measuredconductivity range from 4×10⁻⁴ Scm⁻¹ to 9×10⁻⁴ Scm⁻¹ in the 90-120° C.temperature range, for a molar ratio of Li ions to ethylene oxide,r=0.02.

In a separate aspect, solid polymer electrolytes which exhibit aconductivity drop upon increase of temperature above a thresholdtemperature value are provided. This intrinsic property of certainpolymer electrolytes can be used to prevent thermal runaway in lithiumbattery cells. The term “prevent”, as used herein, means “to decreasethe probability of thermal runaway failure in a battery cell”, and isnot intended to mean that the described polymers entirely eliminate thepossibility of thermal runaway under all conditions.

It was unexpectedly discovered that certain Li salt doped blockcopolymers, such as linear block copolymers described above, exhibit adrop in conductivity when the temperature of polymer electrolyteincreases above a threshold value. The conductivity drop was found to becaused by precipitation of dissolved lithium salt, which occurs above athreshold temperature. This behavior is entirely unexpected, because itis typical for the conductivity of salt-doped polymers to increase withthe temperature increase. For example, conductivity of Li salt doped PEOpolymer increases with increasing temperature, without exhibiting adrop, and without any indication of lithium salt precipitation withtemperature increase. Remarkably, in block copolymers described herein,the conductivity increases until a threshold temperature value whichlies in a range of between 90° C. and 150° C., and starts to drop afterthe threshold temperature value has been reached. In some embodimentsconductivity drop is at least 2-fold, or at least 5-fold, or even atleast 10-fold. For example, the conductivity can increase until athreshold temperature of 110° C. is reached. Above this temperature, theconductivity can drop by an order of magnitude because of saltprecipitation. The conductivity drop typically occurs at least within 30minutes of exceeding the threshold temperature.

A number of block copolymers described above were found to exhibit thisinteresting and valuable property. While it is preferable to usecopolymers having improved structural properties as described in theprevious sections, the conductivity drop is considered to be anintrinsic property of certain block copolymer systems, and is notlimited to block copolymers having high elastic modulus.

While many of the copolymers described above exhibit the conductivitydrop, it is particularly pronounced in copolymers having cylindricalnanostructure morphology (such as illustrated in FIG. 2A). At least somelamellar samples also exhibit the drop, but sometimes to a lesserdegree. The drop is also particularly pronounced in block copolymershaving a volume fraction of conductive block in the range of 0.2-0.6,more preferably, of 0.2-0.5, such as 0.25-0.35. Further, theconductivity drop, in some embodiments, was found to be pronounced inrelatively high molecular weight block copolymers, e.g., in blockcopolymers having molecular weight of greater than 200 kg/mol, and evengreater than 300 kg/mol. For example, SEO samples SEO (54-23), SEO(216-102), and SEO (360-165) having cylindrical ordered nanostructureand having volume fraction of conductive EO phase 0.28, 0.3, and 0.3respectively, all exhibited a conductivity drop of at least 2-fold(e.g., a 10-fold drop) upon reaching a threshold temperature in a rangeof 90-120° C. In SEO (54-23), the first value (54) refers to a molecularweight of polystyrene structural block, while the second value (23)refers to molecular weight of conductive PEO block, where both valuesare given in kg/mol. Other polymer samples are labeled using ananalogous scheme.

In one aspect, a method of screening block copolymers for a blockcopolymer electrolyte suitable for preventing thermal runaway in a cell,is provided. The method involves providing a plurality of blockcopolymers doped with a lithium salt (such as LiTFSI or other saltmentioned above), and measuring the conductivity dependence of theformed solid electrolyte versus temperature. The measurements willpreferably, include at least a portion of the 90° C.-150° C. temperaturerange. For example conductivity dependence can be measured in a rangefrom 25° C. to 150° C. The plurality of block copolymers can include anyblock copolymer without limitation, but, in some embodiments, includesonly linear block copolymers having linear conductive and structuralphases, such as polymers described above. Further, preferably, but notnecessarily, all block copolymers used for screening should have goodstructural properties, such as high elastic modulus, e.g., a modulus ofgreater than 100 MPa. The measurement of conductivity vs. temperaturedependence is performed individually for each polymer and can beperformed either serially or in parallel. Such measurements can beperformed, for example, using ac impedance spectroscopy on a Solartron1260 frequency response analyzer available from Solartron Analytical ofFarnborough, UK.

Based on the results of this measurement, copolymers exhibiting aconductivity drop at a threshold temperature are identified, and areselected as candidates for solid polymer electrolytes suitable forthermal shutoff of a Li battery cell. In some embodiments, copolymersexhibiting at least a 2-fold conductivity drop, or at least a 5-foldconductivity drop, or at least a 10-fold conductivity drop are selected.Further, preferably, polymers having high conductivity (e.g., at least1×10⁻⁵ Scm⁻¹ prior to the conductivity drop) and good structuralproperties (as previously described) are selected. In some embodimentsonly polymers having high conductivity and good structural propertiesare screened.

Table 1 illustrates linear SEO block copolymers which were screened forconductivity drop by measuring the dependence of conductivity ontemperature. The conductivities were measured during heating from 25 to120° C. Of the 13 samples that were screened, five samples exhibitedconductivity drops greater than about 2-fold. “About” as used hereinrefers to a range which includes ±0.5 interval around the recited value.The molecular weights for PS and PEO fraction, a volume fraction of EOin the polymer, and the conductivity drop data are shown in Table 1.

TABLE 1 Block Copolymers screened for conductivity drop Mn Mn VolumeSAMPLE (kg/mol) (kg/mol) Fraction Conductivity Threshold ID PS PEO of EOdrop (fold) Temperature SEO-1 36.3 24.6 0.38 SEO-2 39.7 31.3 0.42 SEO-318.8 29.1 0.58 SEO-4 39.6 53.6 0.55 SEO-5 74.0 98.0 0.55 SEO-6 16.2 16.30.48 about 4 100° C. SEO-7 52.9 67.6 0.54 SEO-8 37.0 25.4 0.385 SEO-953.5 22.9 0.281 about 10 100° C. SEO-11 6.3 7.2 0.519 about 3  90° C.SEO-15 246.8 116.1 0.3 about 2-3 120° C. SEO-16 352.1 165.6 0.3 about3-4 110° C. SEO-17 178.6 199.9 0.5

Of the polymers listed in Table 1, SEO9, SEO15, and SEO16 had acylindrical morphology. The remaining polymers which showed aconductivity drop had a lamellar morphology.

An alternative method for identifying polymers that are suitable forpreventing thermal runaway in batteries involves screening salt-dopedpolymers for evidence of salt precipitation occurring with temperatureincrease. In one embodiment, a plurality of salt-doped polymers arescreened using SAXS. A SAXS profile for each polymer is measured atseveral temperatures, starting at lower temperature, and up to e.g.,120-150° C. Precipitation of salt (which correlates with conductivitydrop) manifests itself in a SAXS profile by increase of intensity inlow-q scattering. The samples which exhibit increase in low-q scatteringwith increase of temperature are identified as candidate polymers forpreventing thermal runaway in lithium batteries.

In another embodiment screening for salt precipitation is performed bymeasuring light scattering of a plurality of samples. Salt precipitationmanifests itself by an increase in light scattering. Similarly to theSAXS-based screening method described above, a plurality of salt-dopedpolymers are subjected to light scattering measurement. Light scatteringfor each polymer sample is recorded at several temperatures, typicallystarting at lower temperature, and increasing the temperature to, e.g.,120-150° C. Those polymers which exhibit increase in light scatteringafter the temperature reaches a threshold value, are then selected.

Electrolyte Fabrication

In one aspect, a method of making a polymer electrolyte in accordancewith the present invention comprises, in an oxygen and moisture freeenvironment, forming a linear block copolymer having a Li ion conductivelinear polymer block with a molecular weight of at least 5000 Daltonsand a structural linear polymer block with an elastic modulus of atleast 1×10⁷ Pa; and incorporating a Li salt into the linear blockcopolymer, such that the resulting polymer electrolyte has an ionicconductivity of at least 1×10⁻⁵ Scm⁻¹. Care must be taken in themanufacture of the linear block copolymer electrolytes to preventadverse reaction of the Li salt and to obtain the polymer product withthe advantageous performance noted herein.

A block copolymer electrolyte in accordance with the present inventioncan be made by synthesizing the structural polymer block by livinganionic polymerization. Then, a monomer of the conductive polymer blockand a crypt and catalyst are added to the structural polymer blockliving anionic polymerization mixture. The polymerization of theconductive block is then allowed to proceed. The reaction may proceed tocompletion and, after it is terminated with a suitable reagent, theresulting diblock copolymer product is precipitated and freeze-dried.Other living polymerization methods like cationic and radicalpolymerization techniques can also be used to synthesize the blockcopolymers.

To render the copolymer conductive to Li ions, a Li salt is blended withthe freeze-dried diblock copolymer using a moisture-free solvent suchthat the Li salt is dissolved with the conductive polymer block. Theresulting polymer/salt solution is then freeze dried to remove thesolvent. The freeze-dried dry polymer/salt mixture can then be subjectedto heat and pressure (compression molded) to form a freestanding polymerelectrolyte film.

A detailed description of a method of making a linear diblock copolymeras applied to a specific embodiment follows. The following example willbe described using SEO; however, it will now be realized that othermaterials and preparation methods may be used. As such, the followingexample is discussed for exemplary purposes only and is not intended tobe limiting.

Synthesis of PS-b-PEO

In an exemplary embodiment of the invention, the diblock copolymer,polystyrene-b-poly(ethylene oxide) (SEO) is synthesized using livinganionic polymerization to attain good control of the molecular weightand achieve low polydispersity. All of the synthetic steps are performedon a high-vacuum line and inside an argon glove box to ensure an oxygenand moisture free environment.

The first step is to purify the solvent, benzene, over calcium hydridefor at least 12 hours at room temperature. Benzene is then distilledonto a sec-butyllithium stage for further purification.

The next step is purification of the styrene monomer, and synthesis ofpolystyrene. The styrene monomer is purified on calcium hydride (CaH₂)by stirring for a few hours to remove trace amounts of water. Themonomer is then distilled into a flask (e.g., short-neck round-bottomflask) containing dibutyl magnesium (DBM). The monomer is stirred overDBM overnight. A clean reactor, in which polymerization will take place,is placed on the high-vacuum line and flamed with a torch to removeadsorbed water and solvent. After flaming, about 20 mL of the purifiedsolvent is distilled from the sec-butyllithium stage into the cleanreactor. This is done to prevent freezing and degassing a large amountof solvent. After the addition of 20 mL of benzene, the reactor is takenoff the vacuum line, and into the glove box at which point thecalculated amount of sec-butyllithium initiator required for reaction ispipetted into the reactor.

A very good seal is maintained during transfer back onto the vacuumline. After the reactor is placed back on the line, the mixture isfrozen with liquid nitrogen and degassed 1, 2, 3, 4, or 5 times toremove trapped argon from the vessel and promote fast distillation. Theremainder of the solvent can then be distilled into the reactor. In onearrangement, enough solvent is added to yield a mixture of approximately8-10 wt % polymer in solution.

The first block, polystyrene (seen as 12 in FIGS. 2A-B, and 16 in FIG.2C) is synthesized by distilling a calculated amount of styrene from theDBM stage into a graduated ampoule and then into the reacting vessel,which contains benzene and the sec-butyllithium initiator. The reactionis carried out at room temperature for approximately 12 hours and acharacteristic yellow “living” polymer mixture is obtained uponcompletion. The mixture is stirred continuously during the reaction toensure uniform mixing of the monomer. The reactor is then taken into theglove box and an aliquot is taken out and terminated with degassedmethanol or isopropanol for characterization using gel permeationchromatography to determine its molecular weight.

This completes the synthesis of the first block, ready for the additionof the ethylene oxide (EO) block. A flask (e.g., short-neck round-bottomflask) with freshly powdered calcium hydride is placed on the vacuumline and opened up to vacuum to remove all air. The flask is maintainedat 0° C. Ethylene oxide monomer, a gas at room temperature (b.p. 10° C.)is condensed onto the calcium hydride at 0° C. and left to stirovernight to remove residual moisture. The monomer-containing flask ismaintained at a dry ice/IPA mixture temperature. The ethylene oxide iskept cold at all times during purification. The ethylene oxide/calciumhydride mixture is frozen and degassed twice before the purificationsteps.

The monomer is purified with two stages of n-butyllithium by stirringfor 30 minutes at 0° C. at each stage. 10 mL of n-butyllithium incyclohexane is used for each stage. The purifying agent, n-butyllithium,is completely dried by pulling vacuum to get rid of the cyclohexanesolvent in which the n-butyllithium is dissolved. This can preventcontamination and ensures an accurate measurement of the amount ofethylene oxide (EO) monomer. The ethylene oxide monomer is distilledinto a graduated ampoule maintained at 0° C. after purifying at bothstages of n-butyllithium.

Following purification and distillation into a graduated ampoule, 1 mlof the EO monomer is distilled into the living polystyrene reactor foraddition of a single EO unit. Only one EO unit is added to the livingpolystyrene chain because of the strong complexation of the lithiumcation to the oxygen in the ethylene oxide unit. The Li cation shows astrong association with the ethylene oxide unit and, thus, nopropagation reaction can occur. To polymerize the second block, acryptand catalyst (e.g., tert-butyl phosphazene base1-tert-Butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)-phosphoranylidenamino]-2Λ⁵,4Λ⁵-catenadi(phosphazene) (tBu-P₄), available from Aldrich. Milwaukee,Wis.) is added to the reaction mixture (containing living EO cappedpolystyrene chains) to complex the lithium ions and facilitate thepolymerization of ethylene oxide. To prepare the catalyst base, thesolvent in which it is dissolved is removed and a known amount ofbenzene distilled into the flask. This solution can then be pipettedinto the reactor in the glovebox. The reactor is then reattached to thehigh-vacuum line and degassed 1, 2, 3, 4, or 5 times to remove trappedargon from the vessel and promote fast distillation. The remainder ofthe EO is then distilled from the graduated ampoule into the reactor.

The reaction is allowed to proceed for three to four days at 45° C.during which time the color changes to a dark blue. The diblockcopolymer (SEO) is then terminated with methanol in the glove box andpurified by precipitation in cold hexane. In those instances, whenalkoxy-terminated polymer is desired, the reaction is quenched with analkylating agent, such as alkyl iodide (e.g., methyl iodide). Theprecipitated polymer is dissolved in benzene and filtered through a 0.2μm filter. The filtered polymer is then freeze dried to remove allsolvent. The freeze-dried polymer is characterized by gel permeationchromatography (GPC) to determine the molecular weight and by nuclearmagnetic resonance (NMR) to calculate the volume fraction of each block.

Electrolyte Preparation

Polymer electrolytes are prepared by blending the freeze-dried SEOcopolymers with the lithium salt, LiN[SO₂CF₃]₂ (LiTFSI, available fromAldrich) in a moisture free environment. This is achieved in a fewsteps. A LiTFSI/THF solution (about 10% w/w) is prepared in the glovebox, and stored as a stock solution. All the copolymer samples areweighed and vacuum dried in a heated antechamber and brought into theglove box. The dry copolymer samples are then dissolved in dry benzene.To this copolymer/benzene solution, the necessary amount of the LiTFSIstock solution is added in the argon glove box. All solvents used in theelectrolyte preparation are doubly-distilled to remove trace amounts ofmoisture. The polymer/salt solution is then freeze-dried in a glove boxcompatible desiccator for one week to remove all solvent. Thefreeze-dried polymer/salt mixture is loosely packed into a pellet in adie that has a diameter slightly less than the spacer inner diameter.The pellet is placed in a Teflon™ bag along with a spacer, such that thepellet is in the center of the spacer's slot. The spacer is made of anon-conducting reinforced resin, and can withstand high pressure andtemperature without deforming. The pellet is then subjected to a highpressure (about 1 kpsi) at about 100° C. This yields a clear,freestanding polymer electrolyte film with the spacer around it. Thespacer defines the edge and the thickness (and hence the area, and thevolume) of the freestanding polymer electrolyte film, and provides ameans for easy transfer in and out of the membrane-electrode assembly.All the steps are carried out in the glove box.

In a separate aspect, a method of making block copolymers is adjustedsuch that during preparation, the block copolymer with an embedded saltis not exposed to a temperature that is higher than the thresholdtemperature for lithium salt precipitation. This adjustment is made forthose polymers which exhibit conductivity drop above a thresholdtemperature due to salt precipitation. Therefore, in these embodimentsafter the polymer/salt mixture is made and is freeze-dried, thefreeze-dried pellet is subjected to high temperature and pressure toform a freestanding polymer electrolyte film, where the temperature ofthe anneal is adjusted to be below the threshold temperature at whichlithium salt precipitation occurs. Depending on a particular polymer andthe threshold temperature, the anneal is performed below 150° C., orbelow 110° C. In some embodiments, due to these considerations, arelatively narrow temperature range for anneal is found to bepreferable, such as 80-120° C., or 100-110° C. When annealing isperformed above the threshold temperature for lithium saltprecipitation, due to precipitation of the salt, the conductivity ofpolymer electrolyte substantially drops. When annealing is performed attoo low a temperature, the film cannot always properly form to have thedesired nanostructure. In some embodiments the temperature for annealingand/or casting is selected such that it is greater than the glasstransition temperature of the structural block material of the copolymerbut is lower than the threshold salt precipitation temperature. One ofskill in the art will understand how to select a temperature range forannealing a polymer electrolyte sample, given these guidelines.

In a separate, but related aspect, the method of making the polymerelectrolyte film involves casting the polymer at high temperature. Inthis case, instead of freeze-drying, evaporation of the solvent from thesolution of the polymer and the salt dissolved in a solvent, isperformed. Upon evaporation, the polymer is heated and is casted suchthat it adopts a shape needed for the electrolyte film. Similarly to theprevious example, in those embodiments where the polymer electrolyteprecipitates the lithium salt above a threshold temperature, the processis adjusted such that casting and/or evaporation of the solvent isperformed below the threshold temperature of salt precipitation. In someembodiments, these considerations provide a relatively narrowtemperature window for the casting and/or evaporation process.Specifically, many copolymers described herein can be cast only above90-100° C. (or their corresponding Tg temperature), as they are too hardand glassy at lower temperatures. However, above the thresholdtemperature for lithium salt precipitation, the polymer loses a lot ofits conductivity. Therefore, casting is performed at a temperature rangeat which the polymer is sufficiently soft for casting and at which saltprecipitation does not occur. In some embodiments, this temperaturerange is between 90-110° C.

Structure and Performance

While the invention is not limited by any particular theory, it isbelieved that the high elastic modulus, high ionic conductivity blockcopolymers of the present invention are correlated with a particularmorphology. It has been found that for at least some specificembodiments of the invention with advantageous properties, thebicontinuous phases of the polymer block constituents of the diblockcopolymer have a lamellar arrangement and that the Li salt segregatesitself to the ionically conductive block channels. In addition, withincreasing molecular weight of the polymer block constituents, the Lisalt increasingly concentrates itself to a central portion of theionically conductive block channels and ionic conductivity increases.

Further, block copolymers having cylindrical morphology were also foundto be suitable in terms of conductivity and structural properties.Further, the conductivity drop is often more pronounced in sampleshaving cylindrical morphology, which makes them attractive as polymerelectrolytes having an intrinsic property of preventing thermal runawayin lithium battery cells.

As described further below with reference to FIG. 8, the linear blockcopolymer electrolytes of the present invention exhibit good cyclingperformance. No dendrite formation was observed for 80 cycles in DCLi/polymer/Li test cells.

EXAMPLES

The following examples provide details relating to composition,fabrication and performance characteristics of block copolymerelectrolytes in accordance with the present invention. It should beunderstood the following is representative only, and that the inventionis not limited by the detail set forth in these examples.

To characterize the electrolyte, experimental data results were obtainedby methods such as transmission electron microscopy (TEM), small angleX-ray scattering (SAXS), AC impedance spectroscopy, and rheology. ACimpedance spectroscopy measurements were made using a test cell onthermo-stated pressed samples in the glove box, and a Solartron 1260Frequency Response Analyzer machine connected to a Solartron 1296Dielectric Interface. The polymer samples for TEM and SAXS were annealedusing the same thermal history as that used for conductivitymeasurements. The electrolyte samples were kept at 120° C. for 2 hrs,and cooled to room temperature. Thin sections (about 50 nm) wereprepared using an RMC Boeckeler PT XL Cryo-Ultramicrotome operating at−100° C. Imaging was done on a Zeiss LIBRA 200FE microscope operating at200 kV. SAXS measurements were made on hermetically sealed samples. Thedata were collected for various temperatures between 140° C. and roomtemperature during cooling. Rheological measurements were performedusing an ARES rheometer from Rheometric Scientific Inc. with aparallel-plate geometry. Approximately 1 mm thick samples were placedbetween 8 mm plates for SEO, and 50 mm plates for PEO, in a closed ovenwith a Nitrogen (N₂) atmosphere. The ratio of Li ions to ethylene oxidemoieties, r, in the electrolyte was varied from 0.02 to 0.10. Unlessotherwise noted, due to the lack of qualitative changes in propertieswith salt concentration, data was obtained at r=0.02. Alternatingcurrent (AC) impedance and rheological data, obtained from mixtures of a20 kg/mol PEO homopolymer and Li[N(SO₂CF₃)₂], serve as the baseline forevaluating the properties of the composite electrolyte.

Table 2 lists the characteristics of the different kinds of copolymersthat will be discussed:

TABLE 2 Mn (PS) Mn (PEO) d spacing Copolymer g/mol g/mol φ_(EO)Morphology (nm) SEO(36-25) 36400 24800 0.38 Perforated Lamellae 47.9 ±0.8 SEO(74-98) 74000 98100 0.55 Lamellar 101.2 ± 3.4  SEO(40-54) 3970053700 0.55 Lamellar 66.4 ± 1.4 SEO(40-31) 39700 31300 0.42 PerforatedLamellae 44.4 ± 0.6 SEO(16-16) 16200 16300 0.48 Lamellar 30.4 ± 0.3SIEO(11-6-9) 11200 9000 0.31 Lamellar 33.0 ± 0.3

Two different block polymer types were utilized to obtain theexperimental data. The first is polystyrene-block-poly(ethylene oxide)diblock copolymers (SEO) doped with lithiumbis(trifluoromethylsulfonyl)imide, Li[N(SO₂CF₃)₂]. The PS-rich phaseprovides mechanical rigidity, while the PEO phase provides ionicconductivity. The second is apolystyrene-block-polyisoprene-block-poly(ethylene oxide) triblockcopolymer (SIEO). The presence of the polyisoprene rubbery domains isknown to increase the impact strength of polystyrene.

FIG. 3 is a graph illustrating SAXS profiles obtained from SEO/salt andSIEO/salt mixtures. The SAXS data were obtained from SEO/salt andSIEO/salt mixtures with r=0.02 and at a temperature of 90° C. Forclarification, SIEO (11-6-9) (marked A) has a ratio ofpolystyrene-polyisoprene-poly(ethylene oxide) of 11-6-9. SAXS dataobtained from pure SEO and SIEO samples were indistinguishable from thedata shown in FIG. 3. The long range order, as gauged by the sharpnessof the primary and higher order peaks, is better in low molecular weightsamples than in the high molecular weight samples. This is expected dueto the slow diffusion in strongly segregated block copolymers.

The SAXS profiles of SEO(36-25) (marked C) are similar to perforatedlamellar phases, while those from the other samples show the presence ofa lamellar phase. TEM images of SEO(36-25) (not shown for brevity)indicate that the PEO lamellae appear dark and perforated, while the PSlamellae show no evidence of perforations, in agreement with the SAXSdata shown in FIG. 3. Based on the similarity in the SAXS profilesobtained with and without salt, the addition of salt at lowconcentrations to the SEO copolymer may not affect the morphology. TheTEM and SAXS data indicate the absence of an interpenetrating networkphase in the block copolymer electrolytes.

FIG. 4A is a graph illustrating the dependence of ionic conductivity onr and temperature, respectively, in SEO(36-25). FIG. 4B illustrates thedependence of ionic conductivity on r at 100° C. in SEO(74-98). FIG. 4Billustrates that the conductivity has a maximum at r≈0.06-0.09. Thistrend is similar to that observed in PEO-salt homopolymer-based systems.At low salt concentrations, ionic conductivity increases with saltconcentration due to the increase in the number of charge carriers. Athigh salt concentrations, transient cross linking of the polymer chainsand neutral ion pairs result in reduced conductivity. FIG. 4Aillustrates the conductivity at various temperature ranges. Itillustrates that SEO (36-25) was conductive at 90° C. and higher.

FIGS. 5A-5C are graphs comparing ionic conductivity of the compositeelectrolytes as a function of the molecular weights of the PEO blocks.FIG. 5A illustrates a systematic increase in conductivity with themolecular weight of the PEO block (M_(PEO)). The observed trend isopposite to that obtained in pure PEO/salt mixtures where theconductivity decreases with increasing molecular weight and eventuallyreaches a plateau. The conductivity, σ_(PEO), of the PEO homopolymerwith r=0.02 at 90° C. and 120° C. was measured to be 3.7×10⁻⁴ S/cm and5.6×10⁻⁴ S/cm, respectively. The conductivity obtained in the highestmolecular weight sample, SEO(74-98) (marked F in FIG. 3), is comparableto that of pure PEO. Data obtained (not shown) from SEO(74-98) at 100°C. show C peaks at a value of about 10⁻³ S/cm at r=0.085. These valuesof conductivity are adequate for some battery applications. FIG. 5Cillustrates dependence of conductivity on the molecular weight of thePEO block separately for lamellar and cylindrical samples. It can beseen that in lamellar samples the conductivity increases with increaseof molecular weight of PEO and then plateaus above 60 kg/mol. In sampleshaving cylindrical morphology, increase of conductivity with increase inPEO block weight is observed, for all molecular weights screened.

The PEO volume fraction in the composite electrolyte, φ_(PEO), variedfrom 0.38 to 0.55. If all of the PEO channels in the nanostructuredelectrolyte provided conducting pathways for the ions, and if theconductivity of the PEO channels were identical to that of PEOhomopolymers, then the value of the conductivity of a doped SEO sample,σ_(max) would be the product of φ_(PEO)σ_(PEO). The ratio σ/σ_(max) thusnormalizes the measured conductivity data for differences in φ_(PEO).FIG. 5B is a plot of σ/σ_(max) versus M_(PEO). FIG. 5B shows that theionic conductivity of the electrolytes may be mainly affected by M_(PEO)and not by block copolymer composition.

FIGS. 5A and 5B illustrate that: (1) it is possible to makeself-assembled conducting nanostructured electrolytes withnon-conducting matrix phases, and (2) the magnitude of the conductivityof such electrolytes is in the range of the theoretical upper limit thatcan be expected from such systems. The samples were not subjected to anyspecial processing steps to ensure that the PEO lamellae were connectedor aligned. Connectivity of the PEO phase occurs reproducibly byquiescent annealing at 120° C. In other embodiments, annealing isperformed at lower temperatures to avoid salt precipitation within thepolymer. Although it has been widely understood that highly connectednetwork phases are essential for high conductivity, the above dataindicates that that is not the case.

FIG. 5B illustrates the fact that σ/σ_(max) for the highest molecularweight sample is close to the theoretical maximum of 0.67 expected for astructure comprised of randomly oriented grains. It is believed thatdissociated Li ions are tightly coordinated with the ether linkages inPEO, and thus disruption of this coordination could lead to faster iontransport. The disruption of coordination may be due to two reasons: (1)the contact between the ions and the polystyrene/poly(ethylene oxide)interfaces, and (2) the deformation of PEO chains due to self-assembly.The polystyrene/poly(ethylene oxide) interfacial area (per unit volume)decreases with increasing molecular weight. As noted above withreference to FIG. 3, potential reason (1) appears incorrect. As such,ion transport within microphases may be faster than in bulk.

In contrast, it is known that block copolymer chains stretch when theyform ordered phases due to the traditional balance of energy andentropy. On average, PEO chains are stretched in the lamellae, comparedto the homopolymer. The extent of stretching depends on copolymer chainlength, N, and the Flory-Huggins interaction parameter between theblocks, χ. The range of the product χN in the SEO diblock copolymersranged from 25 to 130 at 90° C., indicating that these copolymers inaccordance with the invention are far from the weak segregation limit.Another indication of the stretched nature of the PEO chains is thescaling of the PEO lamellae thickness, d_(PEO), calculated from theperiodic length scale, d, given in Table 1 with N_(EO), the number of EOunits in PEO block. A least squares power law fit through the d versus Ndata yields d_(PEO)=0.18*N_(EO) ^(0.69) (both the prefactor and exponentwere free parameters in the fit), which is the expected result in thestrongly segregated limit. Thus, the stretched PEO chains in the highmolecular weight block copolymers are not as tightly coordinated with Liions compared to PEO random coils, which leads to enhanced conductivity.

FIG. 6 is a graph illustrating the frequency dependence of the storageand loss shear moduli. The graph shows the frequency (ω) dependence ofthe storage and loss shear moduli, G′ and G″, respectively, of pureSEO(36-25), SEO(36-25) with r=0.020, and pure PEO homopolymer. The dataobtained from the pure SEO(36-25) and SEO(36-25) with r=0.020 areindistinguishable. The frequency-independence of the moduli and the factthat G′ is an order of magnitude larger than G″ indicate that the blockcopolymer electrolytes are elastic solids. The data also indicates thatthe addition of small amounts of salt has no detrimental effect on themechanical properties of the materials. The value of G′ obtained fromthe SEO(36-25) electrolyte is 100 times larger than the plateau modulusof pure PEO and 6 orders of magnitude larger than G′ of the PEOhomopolymer. The molecular weight of the PEO homopolymer is similar tothat of the PEO block in the SEO(36-25) copolymer. Thus, nanostructuredelectrolytes have roughly half the conductivity of homopolymer PEO butlarger shear moduli by several orders of magnitude.

FIG. 7 is a graph illustrating the frequency dependence of elasticmoduli for the diblock copolymers referenced in FIG. 3. The highermolecular weight copolymers that exhibit high ionic conductivity yieldelastic moduli of the order of 10⁸ Pa. By adding the PS block to the PEOchain, the shear modulus increases by several orders of magnitude whilekeeping the ionic conductivity near the level of pure PEO.

FIG. 8 plots the results of cycling a Li/SEO(74-98) r=0.02/Li cell witha 250 μm thick electrolyte at 90° C. The cell was formed with afreestanding polymer electrolyte film formed as described above herein.A spacer with the freestanding polymer electrolyte film was thensandwiched between two aluminum masks that defined the exposedelectrolyte area available for lithium (Li) deposition. This assemblywas then secured to a slotted disk. The slots in the disk ensure thatthe electrolyte surface was exposed for Li deposition. The slotted-diskwith 30 electrolyte/mask assemblies was then secured to a spindle in theLi deposition chamber. The spindle was connected to a motor that canrotate the spindle, along with the slotted plate with electrolyte/maskassemblies. Once the slotted plate was secured to the spindle, Lipellets were placed in the crucible of the vapor-deposition chamber. Thedeposition chamber was then closed and left overnight under a very highvacuum (<10⁻³ mbar). The high vacuum ensures that the electrolyte and Lipellets are free of any adsorbed solvent molecules that may react withLi vapors. When the pressure in the deposition chamber was below 10⁻³mbar, the crucible was heated to ˜200° C., and Li was deposited from Livapors onto the exposed surface of the electrolyte. The thickness of thedeposited Li layer/film was monitored, and vapor deposition was stoppedonce a desired (˜1 μm) Li-layer thickness was achieved. Then thedeposition chamber was opened and the slotted plate was flipped andsecured back to the spindle, exposing the other face of the freestandingelectrolyte film that had not yet received any deposited Li. Thedeposition chamber was closed and the process of Li-deposition wasrepeated. This yields symmetric Li/SEO/Li cells for performing DCcycling measurements. In each cycle, a 50 μA/cm² current density wasapplied to the cell for 2 h, followed by a rest period of 1 h, followedby a 50 μA/cm² current density applied in the opposite direction. Thevoltage required to achieve this current density was independent of timeover 80 cycles, indicating the lack of dendrite growth during theseexperiments.

The structure of the block copolymer electrolytes of the presentinvention was investigated to determine if a structural correlation forthe observed properties could be determined. Poly(styrene-block-ethyleneoxide) (SEO) copolymer electrolytes were prepared as described above.Samples were pressed at 120° C. into 0.1-1 mm thick disks using amechanical press. All sample preparation was done without exposing thematerials to atmospheric water or oxygen.

Electron microscopy images were analyzed to determine the width of thelithium and poly(ethylene oxide) channels. Micrographs were segmentedinto boxes that were approximately two times the periodicity of thestructures. The minimum periodicity, d_(min), was taken to be the truespacing of the nanostructured electrolyte. Regions which show aperiodicity within 10% of d_(min) were selected for furtherquantification. The intensity distribution of these images wasnormalized to maximize the dynamic range of contrast. A high-pass filterwhich removes low-frequency data below a radius of 30 pixels was used toremove some of the noise from the images. The image was thresholded,such that pixels are either black or white, to measure systematicallythe lamellae thicknesses. The channel thicknesses were measured by handevery 004d_(min) along the lamellae normal.

FIG. 9A is a bright field image of SEO(16-16)/salt mixture (r=0.085).The amorphous nature of the polymer is clearly visible. FIG. 9B is thelithium energy filtered micrograph of the same region as FIG. 9A. Thelight regions correspond to the presence of lithium, and the darkregions to the absence of lithium. Since the salt segregates itself tothe PEO channels, the microstructure is now clearly visible and it ishighly suggestive of a lamellar morphology.

Measurements of the thickness of the lithium salt and poly(ethyleneoxide) lamellae are shown in FIG. 10 for SEO(74-98) at a saltconcentration of r=0.085. The first column corresponds to the product ofthe periodicity observed in our images and the volume fraction ofethylene oxide in the copolymer. The second column is the thicknesses ofPEO lamellae measured from bright field images of neat SEO copolymersstained with RuO₄. The last three columns correspond to lamellaemeasurements taken on energy filtered TEM images of O, Li and F. Thefirst three columns are a measure of the PEO lamellae thickness, whilethe last two are a measure of the Li channels. It is clear from FIG. 10that the salt lamellae are thinner than the PEO channels for SEO(74-98)at a salt concentration of r=0.085. The size of the Li domains werenormalized by the ethylene oxide domains to determine the extent of thisconcentration effect as a function of the molecular weight of PEO,M_(PEO), and these results are plotted in FIG. 11. It is clear that thethinning of the lithium lamellae is more dramatic for polymers withhigher molecular weight.

FIG. 12 is a comparison of the lithium salt distribution and the ionicconductivity of the copolymer electrolytes. There is a clear inverserelationship between the extent to which the Li segregates away from theinterface and the ionic conductivity. It is concluded that byconstructing SEO copolymer/salt mixtures where M_(PEO) is high, a novellithium salt structure within the nanostructured copolymer, with Li saltconcentrated in a central portion of the ionically conductive polymerblock, is produced, which results in highly conductive materials.

FIG. 14 illustrates dependence of conductivity on inverse temperature(1000/T) in a SEO (54-23) sample, having lithium salt concentrationr=0.05. Heating and cooling plots are shown with arrows. It can be seenthat during heating the conductivity increases with the increase intemperature until a threshold of 100-110° C. is reached. Above thistemperature range, the conductivity stops rising and eventually drops byalmost an order of magnitude. Interestingly, during cooling theconductivity does not rise to the initial high value. Therefore, itappears that at least in this sample, and at the time scale used duringmeasurement (about 30 minutes for each data point), the saltprecipitation is irreversible.

FIG. 15 provides evidence that the observed conductivity drop is due tolithium salt precipitation. FIG. 15 provides SAXS plots for the same SEOpolymer. SAXS profiles at 53° C., 88° C., and 134° C. in a heatingregime. The 134° C. temperature value is above the threshold temperaturefor conductivity drop in this polymer. It can be observed that theprofile at this temperature exhibits increased low-Q scattering, whichis a sign of macrophase separation (salt precipitation). The profiles atlower temperatures do not show signs of salt precipitation.

Nanostructured polymer electrolytes with soft conducting channelsembedded in a hard insulating matrix are beneficial for applicationsthat require independent control over mechanical and electricalproperties. The data presented above indicate that network phases arenot necessary to obtain ionic conductivity, which allows for thepossibility of using a wide variety of morphologies for designingionically conducting polymers. No special processing is needed to createpercolating conducting pathways in these materials; they are formedentirely by quiescent self-assembly. This suppresses dendrite growth inLi-polymer batteries and provides a cost-effective solution forcombining high energy density with good cyclability.

Applications

The electrolytes of the present invention are useful as solid phaseelectrolytes for high energy density, high cycle life batteries that donot suffer from failures due to side reactions and dendrite growth onthe Li electrodes. In one aspect, the invention relates to a batterycell, a simple schematic diagram of which is provided in FIG. 13,comprising a lithium anode, a cathode, and a solid phase block copolymerelectrolyte in accordance with the present invention disposed betweenthe anode and cathode. The cell can be cycled without detrimentaldendrite growth on the anode.

In one aspect the battery cell includes a polymer electrolyte which isconfigured for preventing thermal runaway in the battery cell. Thepolymer electrolyte has a lithium salt dissolved therein, and isconfigured for precipitating this salt once the temperature ofelectrolyte exceeds a threshold value. In some embodiments, thisthreshold value lies in the range between 90° C. and 150° C., e.g.,between 100° C. and 110° C. The precipitation of lithium salt leads to adrop or at least a plateau in conductivity of the polymer above thethreshold temperature, which, in turn, leads to cooling, or at leastreduced heating of the cell.

Advantageously, the precipitation of lithium salts in the blockcopolymers described herein is an intrinsic unexpected property of thecopolymers, related to the thermodynamics of the copolymer/salt system.Therefore, in some embodiments the electrolyte itself provides a thermalshutoff mechanism for the lithium battery cell, without the need forcomplicated mechanical shutoff designs. In other embodiments, bothshutoff designs may complement each other and be used in concert.

Preferably, the polymer electrolyte is configured to have a conductivitydrop and a threshold temperature which are sufficient to prevent meltingof lithium metal in the battery cell. For example, polymer electrolyteshaving a threshold temperature at which conductivity drops or plateaus(or lithium salt precipitates) well below the melting point of Li metal(180° C.) are preferred.

Further, in another aspect, a method of using a lithium battery cell isprovided. The cell includes a lithium anode, a cathode, and a solidblock copolymer electrolyte having a lithium salt dissolved therein. Theblock copolymer electrolyte is configured for precipitating the lithiumsalt above a threshold temperature (e.g., above a temperature from therange of 90° C.-150° C.). The method of operating a cell involvescharging and discharging the rechargeable cell. The cell is configuredfor cooling or for reduced heating once the electrolyte reaches thethreshold temperature during battery cell operation. As it wasdescribed, the reduced heating or cooling is due to conductivity drop inthe electrolyte which occurs above the threshold temperature upon saltprecipitation.

The invention may also be used, for example, as a power source forelectronic media such as cell phones, media players (MP3, DVD, and thelike), laptops and computer peripherals, digital cameras, camcorders,and the like. It may also be beneficial for applications in electricvehicles and hybrid electric vehicles.

In addition, as it was previously mentioned, it has been discovered thatcareful processing of polymer films needs to be performed in order toprevent salt precipitation during fabrication of the polymer films. Thechoice of processing temperatures used in annealing and/or castingoperations will depend on the polymer system, specific polymer chainarchitecture, the type of salt, the salt concentration and otherparameters. One of skill in the art will understand how to selectoptimal temperature window for polymer film fabrication, based onprovided considerations and guidelines.

CONCLUSION

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. It should be noted that there are many alternative waysof implementing both the process and compositions of the presentinvention. Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein, but may be modified within the scope andequivalents of the appended claims.

1. A polymer electrolyte, comprising: a linear block copolymer having, aLi-ion conductive non-crosslinked linear polymer block with a molecularweight of at least 5000 Daltons; a structural linear polymer block withan elastic modulus in excess of 1×10⁷ Pa; and an ionic conductivity ofat least 1×10⁻⁵ Scm⁻¹. 2-20. (canceled)
 21. The electrolyte of claim 1,wherein the copolymer is a triblock copolymer ofpolystyrene-block-polyisoprene-block poly(ethylene oxide) (S—I-EO)triblock copolymer.
 22. The electrolyte of claim 1, wherein the blockcopolymer is a polystyrene-block-poly(ethylene oxide) (SEO) diblockcopolymer.
 23. The electrolyte of claim 22, wherein the poly(ethyleneoxide) block has a molecular weight of at least 50,000 Daltons.
 24. Theelectrolyte of claim 23, wherein the polystyrene block has a molecularweight of at least 50,000 Daltons.
 25. (canceled)
 26. The electrolyte ofclaim 1, wherein the volume fraction poly(ethylene oxide) block isbetween 0.25-0.35.
 27. The electrolyte of claim 1, wherein theelectrolyte is configured to dissolve a lithium salt at a firsttemperature and to precipitate the lithium salt at a higher temperature.28. (canceled)
 29. The electrolyte of claim 27, wherein theprecipitation occurs at the threshold temperature between 100° C. and120° C.
 30. (canceled)
 31. The electrolyte of claim 27, wherein theprecipitation is accompanied by at least 5-fold drop of conductivity ofthe electrolyte comprising a lithium salt.
 32. (canceled)
 33. A methodof making a polymer electrolyte, comprising: in an oxygen and moisturefree environment, forming a linear block copolymer having a Li-ionconductive linear polymer block with a molecular weight of at least 5000Daltons and a structural linear polymer block with an elastic modulus ofat least 1×10⁷ Pa; and incorporating a Li salt into the linear blockcopolymer; wherein the resulting polymer electrolyte has a ionicconductivity of at least 1×10⁻⁵ Scm⁻¹.
 34. (canceled)
 35. The method ofclaim 33, further comprising: after incorporating the Li salt into theblock copolymer, exposing the block copolymer to a temperature above 25°C. in an annealing and/or casting process, wherein the annealing and/orcasting process is performed below a threshold precipitation temperaturewithout causing Li salt precipitation.
 36. The method of claim 35comprising performing annealing and/or casting at a temperature below150° C.
 37. (canceled)
 38. The method of claim 35, wherein the blockcopolymer is a polystyrene-block-poly(ethylene oxide) (SEO) diblockcopolymer, and wherein the annealing and/or casting is performed at atemperature range of between 90-110° C.
 39. The method of claim 33,wherein the method comprises: synthesizing the structural polymer blockby living anionic polymerization; adding a monomer of the conductivepolymer block and a cryptand catalyst to the structural polymer blockliving anionic polymerization mixture; allowing a diblockcopolymerization reaction to proceed; terminating the reaction;precipitating and freeze-drying the resulting diblock copolymer product;blending the freeze-dried diblock copolymer with the Li salt in a drysolution such that the Li salt is dissolved into the conductive polymerblock; and freeze-drying the polymer/salt solution.
 40. The method ofclaim 39, further comprising subjecting a portion of the freeze-driedpolymer/salt solution to heat and pressure to form a freestandingpolymer electrolyte film. 41-44. (canceled)
 45. The method of claim 33,wherein incorporating a Li salt into the linear block copolymercomprises blending solutions of the lithium salt and the linear blockcopolymer in one or more liquid solvents, and subjecting the formedsolution to heat to evaporate the one or more liquid solvents and tocast a copolymer film of desired dimensions, wherein said subjecting toheat dose not expose the formed solution to the threshold temperatureabove which lithium salt precipitation occurs.
 46. A battery cell,comprising: a Li anode; a cathode; and a solid phase polymer electrolyteas claimed in claim 1 disposed between the anode and cathode. 47.(canceled)
 48. The cell of claim 46, wherein the cell is configured forthermal run-away shutoff by providing a conductivity drop in the linearblock copolymer responsive to a temperature increase of the blockcopolymer beyond a threshold temperature.
 49. The cell of claim 48,wherein the conductivity drop is at least 5-fold. 50-59. (canceled) 60.A method of operating a battery cell equipped with a thermal run-awayshutoff, the method comprising: (a) providing a battery cell comprisinga Li anode, a cathode, and a solid block copolymer electrolyte having alithium salt dissolved therein, wherein the block copolymer electrolyteis configured for precipitating lithium salt above a thresholdtemperature; (b) charging the battery cell; and (c) discharging thebattery cell, wherein the battery cell is configured for cooling or forreduced heating once the electrolyte reaches the threshold temperatureduring battery cell operation, wherein said cooling or reduced heatingis due to conductivity drop in the electrolyte associated with lithiumsalt precipitation.
 61. (canceled)
 62. The method of claim 60, whereinthe threshold temperature is between 100° C. and 120° C.
 63. (canceled)64. The method of claim 60, wherein the block copolymer electrolytecomprises polystyrene-block-poly(ethylene oxide) (SEO) di-blockcopolymer wherein the volume fraction of poly(ethylene oxide) block isbetween 0.2 and 0.6.
 65. (canceled)
 66. A method of screening blockcopolymer electrolytes for a block-copolymer electrolyte suitable forpreventing thermal runaway in a cell, the method comprising: (a)providing a plurality of block-copolymers having a lithium saltdissolved therein; (b) measuring dependence of conductivity, SAXS, orlight scattering profiles versus temperature for at least some of theprovided block copolymers; (c) based on the measurements obtained in(b), identifying the block copolymers which exhibit a drop inconductivity or exhibit evidence of salt precipitation in the SAXS orlight scattering profile with increase of temperature above a thresholdtemperature.
 67. The method of claim 66, wherein the block copolymersprovided in (a) have a conductivity of at least 1×10⁻⁵ Scm⁻¹ at least atsome temperatures of the measured temperature range.
 68. The method ofclaim 66, further comprising identifying block copolymers exhibiting atleast 5-fold drop in conductivity at a temperature range of about 90°C.-150° C.